The present invention relates to a method of compensating for the differences in persistence of the phosphors in a colour image display screen. It also relates to a device for controlling the image display screen which implements the method.
The present invention will be described with reference more particularly to display screens consisting of plasma panels which are either of the DC type with memory or of the AC type. However, it is obvious to those skilled in the art that the present invention may apply to other types of image display screens, more particularly colour image display screens, in which several adjacent cells are covered with different phosphors in order to form a colour pixel.
In a known manner, plasma panels are flat-screen display devices which operate on the principle of an electrical discharge in a gas. Plasma panels, or PDPs, generally have two insulating slabs defining a space filled with a gas mixture containing neon and xenon. The slabs support two or more arrays of crossed electrodes. Each intersection of electrodes defines a cell to which a small volume of gas corresponds. Electrical discharges may be generated in each volume of gas by applying suitable voltages to the corresponding two crossed electrodes. The crossed electrodes constitute the lines and columns, respectively, of the display screen. The number of line and column electrodes determines the definition of the screen. Each intersection of a column electrode with a line electrode will correspond to a video cell containing said volume of gas. In the case of a colour-type image display screen, each cell will be covered with a differently coloured phosphor, especially a red, green or blue phosphor, said cells being combined into triplets, each triplet forming a video pixel. Consequently, there are therefore three times as many column electrodes as there are pixels. On the other hand, the number of line electrodes is equal to the number of lines in the panel.
Given this matrix-type architecture, in order to excite a specific video cell and thus obtain a gas in the plasma state at discrete points, it is sufficient to apply a potential difference at the intersection of a line electrode with a column electrode. The ultraviolet rays coming from the excitation of the gas will thus bombard the red, green or blue phosphors and thus turn a red, green or blue cell on. In order to obtain the threexe2x80x94red, green and bluexe2x80x94components of a television-type image, the electrical conditions for the excitation of the cell remain the same. The three components are obtained solely by choosing three different phosphors. As a consequence, in order to achieve good colour homogeneity, it is important to make the proper choice of phosphors. Among the phosphors currently used, a phosphor of composition Mn:Zn2SiO4 is used for the green colour. Unlike the red and blue phosphors, the green phosphor has a persistence of about 28 milliseconds, whereas the persistence of the red phosphor is less than 5 milliseconds and that of the blue phosphor less than 1 millisecond. This persistence of the green phosphor must be put into the context of the frame period which is 20 milliseconds. Consequently, during a white-to-black time transition for example, namely when going from a white frame to a black frame, the green phosphor will continue to emit light for a time longer than one frame period after the transition, as shown in FIG. 1 in which:
curve a) shows the incident video signal;
curve b) shows the response of the red phosphor;
curve c) shows the response of the green phosphor;
curve d) shows the response of the blue phosphor and
the bar e) shows diagrammatically the various grey levels.
This will result in an impression of green afterglow at this transition. The same will apply on going through a black-to-white transition which will result in an impression of magenta afterglow at the transition, as shown in FIG. 2 in which curves a), b), c) and d) and the bar e) have the same meaning as in FIG. 1. However, this persistence of the green will have no effect on homogeneous regions. This is because, since the grey level remains constant, the eye will not perceive any difference.
The object of the present invention is to provide a simple method making it possible to remedy these afterglow defects during large transitions, such as black-white or white-black transitions.
As a consequence, the subject of the present invention is a method of compensating for the differences in persistence of the phosphors in an image display screen consisting of cells arranged in lines and in columns, several adjacent cells being covered with different phosphors in order to form a pixel, the cells of one pixel being put either into an off state or into an on state for a time within one frame period depending on the grey level to be displayed, characterized in that, at the pixel, the transitions between a first grey level and an adjacent second grey level are detected and in that, if the transition is greater than a threshold, the state of the cell covered with a persistent phosphor is forced to the second grey level before the end of the frame period.
According to a preferred embodiment, the transition is detected by comparing the (nxe2x88x921)th frame with the nth frame so as to detect the inter-frame differences greater than said threshold. Moreover, in the case of a plasma panel, each cell is in the off state or in the on state during n successive subscans of different duration distributed over a frame period. In this case, at least the last subscan is forced to the second grey level.
According to a preferred embodiment, when the difference between the nth frame and the (nxe2x88x921)th frame is negative, the last subscan is forced to 0 and when the difference between the nth frame and the (nxe2x88x921)th frame is positive, the last subscan is forced to 1.
According to the present invention, the transitions detected are the strong transitions, namely, at a pixel, the transitions between a white frame and a black frame or between a black frame and a white frame.
The present invention also relates to a device for controlling a display screen which implements the above method.
According to one embodiment of the present invention, the control device comprises a video processing circuit receiving, as input, a video signal and delivering video coding words, the processing circuit having as many elementary processing circuits as there are cells forming a pixel, said cells being covered with different phosphors, a video memory receiving the video coding words and transmitting column-control words to a circuit for supplying the columns of the display screen. It is characterized in that the processing circuit, corresponding to the cell covered with a persistent phosphor, includes means for detecting the transition between a first grey level and a second grey level and means for forcing the signal output to a value corresponding to the second grey level before the end of the frame period.
According to a preferred embodiment, the transition detection means include a circuit for storing the (nxe2x88x921)th frame and a circuit which computes, for each pixel, the difference between the nth frame and the (nxe2x88x921)th frame and sends a control signal when the difference is greater than a threshold. Moreover, each elementary processing circuit carries out a transcoding operation on the video signal in order to deliver the video control words. As a result, the means for forcing the signal consists of a circuit which changes the value of the video control words output by the transcoding circuit depending on the control signal sent by the circuit computing the difference between the nth frame and the (nxe2x88x921)th frame.
According to an additional feature of the present invention, the image display screen is a plasma panel.